Mazatlan packs more variety than most Mexican beach towns. The Centro Historico is a working old town with restored 19th-century buildings around the Plazuela Machado. The Malecon runs for over 20 kilometres along the Pacific. And the headlands and offshore islands at the south end of the city give it a geography that the resort strip up north does not.
You can do the main sights of the old town on foot in a day. The Faro on Crestón Island is a half-day climb. The Zona Dorada is its own world — go there for the beach hotels, not for character.